1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof. More specifically, the present invention relates to a device isolation structure for electrically isolating circuit elements in a semiconductor device, and a method for forming the same.
2. Description of the Related Art
Unit circuit elements, constituting a semiconductor device (e.g., a transistor, diode, capacitor, resistor, or combination thereof), are designed to be electrically isolated so that they are accessed independently of each other. Trench isolation is the representative isolation structure for electrically isolating unit circuit elements. The trench isolation structure comprises an insulating layer filled in a trench that is formed by partially etching a semiconductor substrate, thus defining a plurality of active regions in the substrate. The trench isolation structure helps to electrically isolate circuit elements formed on each active region.
As integration of semiconductor devices increases, distances between circuit elements become shorter. Therefore, electrical isolation of elements is getting more and more important. Especially, in the case where a strained channel having more increased charge-mobility is formed on a semiconductor substrate for the purpose of improvement of a signal transmission speed and a ratio of current to voltage (i.e., conductivity), there is an increased possibility that charges will leak under an isolation structure and adjacent circuit elements will break down.
FIGS. 1 and 2 are cross-sectional view illustrating a method for forming an isolation structure in a strained semiconductor substrate according to the conventional art.
Referring to FIG. 1, a semiconductor substrate 10 comprises a semiconductor layer 12 thereon having a lattice parameter larger than that of the substrate 10. Then, a strained semiconductor layer 14 is epitaxially grown on the semiconductor layer 12, using an element having a lattice parameter smaller than that of the semiconductor layer 12. In the case where an epitaxial layer consisting of relatively small size of atoms is grown on the semiconductor layer 12 consisting of relatively large size of atoms, the lattice parameter between the small atoms is increased to form a strained structure. In general, the semiconductor layer 12 comprises a germanium epitaxial layer having the lattice parameter (e.g., distance between atoms) larger than that of the silicon substrate 10, and the semiconductor layer 14 comprises a silicon epitaxial layer grown on the germanium layer 12. In such case, the distance between silicon atoms in the semiconductor layer 14 is increased to form the strained silicon layer. Because the lattice parameter of the strained silicon layer is larger than that of the silicon substrate, the strained silicon layer has superior charge mobility and conductivity (e.g., a ratio of current to voltage) to typical bulk silicon.
Referring to FIG. 2, when a semiconductor circuit elements are formed on a substrate including a strained semiconductor layer that has higher charge mobility and conductivity, as described above, trench isolation structures 16 may have a depth sufficient to prevent current leakage and reduction of break-down voltages. However, such deep trench isolation structures may hinder filling the trenches with an insulating material, and thus may have voids therein due to a high aspect ratio of the trench structure. As a result, characteristics and reliability of devices may deteriorate.